use exim/postfix/anyMTA to pass mail through smth@server.com

Note: I don't want normal relaying, as I can't configure the server (server.com) to accept relaying

Current situation:My mail server is on a hosting account and I can't configure it's settings.

Problem:I would like to use a locat MTA, such as exim/postfix (prefferably postifix) to send e-mail from my local machine to the outside world, by auth-ing to account smth@server.com. I can spoof the sender (since I'm auth-ed), so.. it would be theoretically possible.

I wasn't able to find a config setting in postfix's vast configuration which would allow such forwarding to account@server.com.